Using the anti-flooding feature of sh404SEF you can limit the rate at which requests from a given IP address are accepted by your web site:
By default, sh404SEF will allow at most
10 requests in a
10 seconds period of time coming from the same IP address. Any additional request will be denied with a 403 Forbidden HTTP status code response.
Access will be granted again as soon as the request rate goes below 10 requests in 10 seconds.